Myotonic dystrophy (type 1) (DM1) is the most prevalent form of muscular dystrophy in adults and is caused by a CTG expansion in the 3' untranslated region of the DMPK gene. Pathogenesis in DM1 is due to a gain of function of the DMPK mRNA that contains expanded CUG repeats (CUGexp RNA). The CUGexp RNA alters the functions of RNA binding proteins such as members of the muscleblind like (MBNL) and CUGBP, Elav-like family (CELF) families. MBNL1 is sequestered on the expanded CUG repeats producing a loss of function while CELF1 is up regulated due to activation of protein kinase C (PKC) leading to its phosphorylation and stabilization. While much has been learned regarding the molecular consequences of CUGexp RNA expression, the mechanisms by which CUGexp RNA causes progressive muscle wasting remains unknown. To study the pathogenic mechanism in muscle, we developed a muscle specific tetracycline inducible transgenic mouse model expressing the last 1200 nucleotides of the DMPK mRNA containing 960 CUG repeats. Animals induced to express the CUGexp RNA develop progressive and severe histopathology and muscle wasting. We will use this model to determine the role of age in disease progression and identify the features of chronic wasting that do and do not regress upon removal of the CUGexp RNA. We will use genetic approaches in the mouse model to determine whether the loss of CELF1 or gain of MBNL1 and/or MBNL2 prevents or rescues the muscle phenotype. We will determine whether the PKC pathway is activated in DM1 skeletal muscle, as has been shown in DM1 heart, and whether PKC inhibition rescues the muscle phenotype. RNA-seq data from muscle of DM1 mouse models and DM1 muscle samples will be used to identify and then test the roles of critical aberrant gene expression and alternative splicing events in muscle pathology. We will also perform studies to identify CELF/MBNL-independent mechanisms of muscle pathology. The goal of this study is to develop an understanding of the molecular mechanisms by which CUGexp RNA leads to progressive muscle loss and thereby increase the number of potential therapeutic targets.